In air fractionation, the ratio of the amount of down stream liquid to the amount of up stream steam in the column is called reflux ratio. If the number of plates is fixed, the purity of distillation will depend on the reflux ratio. For example, when the reflux ratio is large, there will be less rising gas or more rising liquid. When the rising steam with higher temperature and the downflow liquid with lower temperature exchange heat and mass on the tray, their temperature will tend to be the same. After mixing, the temperature tends to be low temperature liquid, and the nitrogen evaporation will be reduced. Because the oxygen component is difficult to volatilize under the same pressure, more will be condensed. As a result, the purity of nitrogen at the top of the upper column will be improved and the purity of liquid oxygen at the bottom of the upper column will be reduced.
